Baths containing heavy metals such as copper, nickel, cobalt and the like in soluble form are widely used commercially in plating, etching and other processes. The disposal of waste from such baths in an environmentally safe manner presents a challenge. The first step of many disposal processes generally involves electrolytic deposition of at least a major portion of the heavy metal content, followed by treatment of the remaining bath liquid to remove other constituents. The removal of heavy metals from waste baths by electrolytic deposition in this manner is referred to hereinafter as electrowinning of the metal.
The treatment of etchant baths containing copper forms a special instance of such an electrowinning process since, in many cases, such baths can be regenerated for further use as etchants by electrowinning of a portion of the copper content therefrom. The etching of copper is a step carried out in a variety of production processes. A particular example is found int he manufacture of circuit boards which generally begins with a non-conducting substrate such as a phenolic or glass reinforced epoxy sheet laminated on one or both sides with a layer of copper foil. An etch resist image in the shape of a desired circuit pattern is applied to the copper foil and the foil so imaged is subjected to the action of an etchant, by spraying or immersion, to remove the copper not covered by the etch resist. The resist-covered copper circuit pattern is thereby caused to stand out in vertical relief.
The etchants most widely used commercially are cupric chloride alkaline ammoniacal solutions because they provide high etch rates. A major drawback of this type of etchant lies int he difficulty of treating and disposing of the waste therefrom. Electrolytic attempts to recycle or regenerate such baths directly have hitherto been largely unsuccessful due to the corrosive nature of the etchant and the large amounts of chlorine gas which are generated.
Efforts have been made to employ cupric sulfate alkaline ammoniacal etchants since these can be regenerated by electrolytic means without generating chlorine gas. However, these sulfate-based baths suffer from low etch rates. Cordani et al U.S. Pat. No. 4,784,785 reviews prior attempts to increase the etch rate of these baths and describes the use of organic thio compounds to accelerate the etch rate. However, the accelerated rate so achieved is still significantly less than that of chloride-based etchants.
Attempts to regenerate chloride-based etchants using processes which do not generate chlorine gas are reviewed in Lee U.S. Pat. No. 4,915,776, the teachings of which are incorporated herein by reference. These various attempts include electrolytic recovery of the copper content by indirect techniques. The '776 patent is also directed to a process of treating spent etchant. The process involves precipitating copper as a copper hydroxide sludge by reaction with calcium hydroxide. The ammonia gas which is also generated in the reaction is then reacted with the aqueous calcium chloride solution (remaining after the precipitation) and carbon dioxide gas to generate an aqueous solution of ammonium hydroxide and ammonium chloride and a precipitate of calcium carbonate. After separation of the latter, the remaining solution is used to formulate a fresh etchant bath. This process requires high initial investment in complex equipment, as well as further treatment to recover metallic copper from the hydroxide precipitate.
Furst et al U.S. Pat. No. 4,564,428 describes a process for regenerating a sulfate-based ammoniacal copper etchant bath by electrolytic means in the presence of a small amount of ammonium chloride. The oxygen generated at the anode is said to prevent evolution of chlorine gas.
It has now been found that heavy metals can be recovered from baths containing the same by electrowinning using a novel bipolar cell having significantly improved efficiency as will be described in detail hereafter. It has been found further that the novel cell in question has the additional advantage in that it can be used to regenerate chloride-based ammoniacal copper etchant baths by direct electrolytic means without generating of any significant amount of chlorine gas. The copper is recovered from the etchant bath in the form of ductile sheets which can be stripped from the cathode.